Ethylene/propylene rubbers (EPM) or ethylene/propylene /diene rubbers (EPDM) are excellent in weathering resistance, ozone resistance and thermal aging resistance, because they have no double bond in a main chain of the molecular structure. Owing to these excellent properties, the ethylene/propylene/diene rubbers have been widely used for various automotive parts where static force is applied, such as weatherstripping, channel for door glass run and radiator hose.
On the other hand, for most of automotive parts requiring mechanical strength against dynamic force, such as tire, rubber vibration insulator and wiper blade, there have been used conjugated diene rubbers having double bond in a main chain of the molecular structure, such as NR, SBR, IR and BR, and blends thereof.
By the way, in accordance with high performance of automobiles in recent years, it has been eagerly desired to improve thermal aging resistance and weathering resistance of automotive parts.
However, the conjugated diene rubbers are poor in heat resistance and weathering resistance though they are excellent in dynamic mechanical characteristics (dynamic fatigue resistance), whereas EPDM is bad in dynamic mechanical characteristics though it is excellent in weathering resistance, ozone resistance and thermal aging resistance.
On that connection, studies on various blends of EPDM and the conjugated diene rubbers have been made by blending EPDM with the conjugated diene rubber to exhibit advantages of these materials.
Techniques with respect to the blends of EPDM and the conjugated diene rubbers in the above studies are listed in "Japanese Rubber Institute Bulletin, 51,685" by Yasuhiro Oda and Masashi Aoshima, 1978. In this bulletin are introduced as blending means (1) polysulfide vulcanization, (2) peroxide vulcanization, (3) use of prevulcanized EPDM, (4) use of EPDM having a high iodine value, (5) use of halogenated EPDM, and (6) use of an accelerator having a long chain alkyl group.
However, any of those means has no practical effect, and almost no product has been produced yet utilizing those means. The reason is that a vulcanization speed of the conventional EPDM is extremely slow as compared with the conjugated diene rubber. That is, because of the slow vulcanization speed of EPDM, a mixture of EPDM and the conjugated diene rubber is insufficient in mutual vulcanization between EPDM and the conjugated diene rubber, and hence mechanical strength of the mixture is deteriorated.
Accordingly, in order to obtain a rubber composition having advantages of both the conjugated diene rubber and EPDM, there has been eagerly desired the advent of a polymer not only showing low-temperature flexibility of the same level as that of the conjugated diene rubber and heat resistance and weathering resistance of the same levels as those of EPDM but also having an extremely high vulcanization speed.
Copolymerization of ethylene with .alpha.-olefin (e.g., propylene) and 7-methyl-1,6-octadiene (also referred to as "MOD" hereinafter) in the presence of a catalyst comprising a vanadium compound which has been conventionally used for preparing an ethylene/propylene/non-conjugated diene copolymer rubber has been tried, but it is impossible to industrially copolymerize these olefins because activities of the catalysts are markedly deteriorated.
Further, a copolymer of ethylene, .alpha.-olefin (e.g., propylene) and MOD obtained by using a catalyst comprising a titanium compound which has been conventionally used for preparing polyethylene or polypropylene has a wide composition distribution and poor vulcanization properties. Moreover, this copolymer can be difficultly prepared in a homogeneous solution because a copolymer having high ethylene content is precipitated during the preparation of the copolymer.
As prior art, Japanese Patent Laid-Open Publication No. 2(1990)-51512 discloses an unsaturated ethylene/.alpha.-olefin random copolymer whose vulcanization speed is higher as compared with conventional unsaturated ethylene/.alpha.-olefin random copolymers. The unsaturated ethylene/.alpha.-olefin random copolymer disclosed in this publication is a random copolymer derived from ethylene, .alpha.-olefin of 3 to 12 carbon atoms and a non-conjugated diene compound represented by the following formula [I]and characterized in that:
1) a number-average molecular weight is in the range of 3,000 to 500,000 in terms of polystyrene;
2) a weight-average molecular weight is in the range of 6,000 to 5,000,000 in terms of polystyrene;
3) a content of the non-conjugated diene compound is not less than 3 in terms of iodine value; and
4) a bonding ratio of ethylene to .alpha.-olefin is 5-90/95-10 (molar ratio). ##STR1## wherein n is an integer of 2 to 10, R.sup.1 is an alkyl group of 1 to 8 carbon atoms, and R.sup.2 and R.sup.3 are the same or different from each other and are each independently a hydrogen atom or an alkyl group of 1 to 8 carbon atoms.
Japanese Patent Laid-Open Publication No. 2(1990)-64111 discloses a process for preparing an EPDM elastomer of low crystallinity and high molecular weight using a catalyst formed from a metallocene compound of titanium, zirconium or hafnium and aluminoxane.
However, the above-mentioned publications do not disclose any ethylene/.alpha.-olefin/7-methyl-1,6-octadiene copolymer rubber having a narrow composition distribution and excellent low-temperature flexibility.
In the light of the foregoing, the present inventors have earnestly studied catalysts, and found that ethylene, .alpha.-olefin and 7-methyl-1,6-octadiene can be copolymerized with high activities when a Group IVB transition metal catalyst containing a specific zirconium catalyst component is used. In the present invention, ethylene is copolymerized with .alpha.-olefin (e.g., propylene) and MOD in the presence of a specific Group IVB transition metal catalyst.
Further, the present inventors have studied on co-catalysts, and found that the copolymer obtained by copolymerizing ethylene, .alpha.-olefin (e.g., propylene) and MOD in the presence of a catalyst system wherein an organoaluminum oxycompound is used as a co-catalyst in combination with a Group IVB transition metal catalyst such as zirconium compound has a narrow composition distribution and excellent low-temperature flexibility. Furthermore, they have also found that when the above-mentioned copolymer is covulcanized with a conjugated diene rubber, this copolymer exhibits excellent covulcanizability with the conjugated diene rubber, and that this copolymer provides a composition capable of forming a vulcanized rubber which is excellent in weathering resistance, ozone resistance, thermal aging resistance and low-temperature flexibility without being deteriorated in the high mechanical characteristics, abrasion resistance and dynamic fatigue resistance inherently belonging to the conjugated diene rubber such as natural rubber (NR), styrene/butadiene rubber (SBR), isoprene rubber (IR) or butadiene rubber (BR).
Thus, the present invention has been accomplished.